Article pubs.acs.org/Langmuir
Multifunctional Surface Manipulation Using Orthogonal Click Chemistry Karson Brooks,† Jeremy Yatvin,† Christopher D. McNitt, R. Alexander Reese, Calvin Jung, Vladimir V. Popik, and Jason Locklin* Department of Chemistry, College of Engineering, and the Center for Nanoscale Science and Engineering, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *
ABSTRACT: Polymer brushes are excellent substrates for the covalent immobilization of a wide variety of molecules due to their unique physicochemical properties and high functional group density. By using reactive microcapillary printing, poly(pentafluorophenyl acrylate) brushes with rapid kinetic rates toward aminolysis can be partially patterned with other click functionalities such as strained cyclooctyne derivatives and sulfonyl fluorides. This trireactive surface can then react locally and selectively in a one pot reaction via three orthogonal chemistries at room temperature: activated ester aminolysis, strain promoted azide−alkyne cycloaddition, and sulfur(VI) fluoride exchange, all of which are tolerant of ambient moisture and oxygen. Furthermore, we demonstrate that these reactions can also be used to create areas of morphologically distinct surface features on the nanoscale, by inducing buckling instabilities in the films and the grafting of nanoparticles. This approach is modular, and allows for the development of highly complex surface motifs patterned with different chemistry and morphology.
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INTRODUCTION The ability to pattern synthetic and biological molecules is useful for designing model surfaces to observe cell−surface interactions.1−3 For instance, the growth of different tissues, such as bone4 or neurons,5 can be guided and studied using patterned surfaces, which is critical in establishing structure/ property relationships in the field of tissue engineering.6,7 Such surfaces are also attractive for integration into multifunctional biosensors, which can be used to monitor biological responses from complex mixtures such as physiological fluids.8 In addition to the chemical content of a surface, the modulus9 and the micro- and nanomorphology of a surface coating have immense impact on the interactions between a live cell and a surface.10,11 Ultimately, with greater control of the spatial, chemical, and morphological complexity of synthetic surfaces, one can begin to mimic biological surfaces such as cell membranes and extracellular matrices.12−15 Reactive polymer brush coatings have become a common platform for the addition of densely packed chemical functionalities onto surfaces through postpolymerization modification, with click-type reactions being especially promising due to their fast kinetics, quantitative yields, and lack of side-reactions.16−18 The specificity inherent in click reactions also allows for the covalent addition of chemistries to the surface in an orthogonal manner from a complex mixture of components in solution. By employing different click reactions in localized areas of a substrate, a surface can be patterned with a variety of chemically distinct molecules.19−27 © XXXX American Chemical Society
In this work, three different surface chemistries have been used to pattern substrates in an orthogonal fashion: aminolysis of activated esters, strain promoted azide−alkyne cycloaddition (SPAAC), and silyl ether sulfur-fluoride exchange (SuFEx).25,28−30 These reactions are premier click-type reactions that can all be performed under ambient conditions with rapid rates at room temperature, do not utilize metal catalysts, and produce zero or only innocuous byproducts. By using reactive microcapillary printing (R-μCaP),25,31 a highly adaptable and simple patterning technique, to add the initial SuFEx and SPAAC functionalities to the surface by aminolysis, trifunctional surfaces can easily be fabricated with high fidelity. To illustrate the versatility of this method, two different types of trifunctional surfaces have been prepared. The first utilizes three different dyes to illustrate the high resolution of R-μCaP functionalization, orthogonal reactivity, and the quantitative attachment of three different molecules to the surface from one solution. The second example uses reactive nanoparticles and stress-induced creasing with a reactive polymer to create three distinct morphological subdivisions on the surface.32 For each trireactive substrate, the surface contains pentafluorophenyl acrylate (PFPA), oxa-dibenzocyclooctyne (ODIBO),28 and an aryl sulfonyl fluoride (PEFABLOC), which can undergo aminolysis, SPAAC, and SuFEx on the surface, respectively Received: April 26, 2016 Revised: June 7, 2016
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DOI: 10.1021/acs.langmuir.6b01591 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. Reaction Scheme for the Formation of a Trireactive Surface, Followed by the Addition of Three Different Molecules to the Surface
permeation chromatography. Gel permeation chromatography (GPC) was conducted on a liquid chromatograph (Shimadzu LC-20AD series) equipped with a RID-10A refractive index detector. Polymer samples were diluted in a THF mobile phase and passed through three Phenomonex Phenogel (10E3A, 10E4A, and 10E5A) columns at 40 °C under a constant volumetric flow rate (1 mL min−1). Molecular weight characteristics of the samples were referenced to polystyrene standards (Agilent Technologies EasiCal PS-2). PDMS Stamp Fabrication. PDMS was made using the SYLGARD 184 silicone elastomer kit from Dow Corning. Microfluidic masks were designed on AutoCAD (Autodesk, Inc., San Rafael, CA) and printed on transparencies at 20000 dpi by CAD/Art services, Inc. (Bandon, ORD). Individual Substrate Fabrication. Poly(PFPA) brushes on silicon wafers were functionalized in 40 mM solutions of 1aminomethyl pyrene, ODIBO-amine, and PEFABLOC for 1 h in DMF with 80 mM of triethylamine. The substrates were then removed, rinsed vigorously with DMF, dried under a stream of nitrogen, and characterized via spectroscopic ellipsometry, drop shape analysis, and FTIR. The substrates functionalized with ODIBO-amine and PEFABLOC were then placed in 13 mmol Texas Red azide in 2 mL DMF and 0.1 mmol TBDMS-fluorescein methyl ester with 0.02 mmol TBD in 2 mL DMF respectively for 30 min. After 30 min, the slides were removed, washed with DMF, dried under a stream of nitrogen, and characterized via spectroscopic ellipsometry, drop shape analysis, and FTIR. UV/Vis Kinetic Traces. PEFABLOC brush functionalized glass slides were measured on a UV−vis spectrometer using a slide holder accessory with a sample window area of 19.6 mm2. The functionalized slide was immersed in a solution of 0.1 mmol TBDMS fluorescein methyl ester silyl ether (the solution is saturated at this concentration) and 0.02 mmol TBD in 2 mL of MeCN for a predetermined amount of time. The slides were rinsed thoroughly with DMF and DCM prior to each measurement. The rate of substitution of fluorescein methyl ester onto the polymer brush was measured by monitoring the appearance of the dye peak with time, and a kinetic plot was generated using the linear portion of the absorbance plot. This procedure was repeated on poly(PFPA) brush with 40 mM 1-aminomethylpyrene with 80 mM triethylamine in DMF and again on an ODIBO brush with 4 mM azido-Disperse Red 1 in DMF. Tripatterned Surface Fabrication. Using a poly(PFPA) brush grafted-from silicon dioxide by surface initiated radical photopolymerization described above, 2 μL of 40 mM ODIBO-amine in DMF with 80 mM triethylamine (TEA) was printed using via
(Scheme 1). The click functionality counterparts to these surface moieties (amines, azides, and silyl ethers) are easily accessible as commercial compounds or through straightforward synthesis from a variety of biological or synthetic substrates.
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EXPERIMENTAL SECTION
Materials. Silicon wafers (orientation ⟨100⟩, native oxide) were purchased from University Wafer. Jeffamine M-2070 was provided as a gift by Huntsman Chemical. All organic solvents were dried and freshly distilled before use. Tetrahydrofuran was distilled from sodium/benzophenone ketyl, and dichloromethane was distilled from CaH2. Other reagents were obtained from Sigma-Aldrich, TCI, or VWR and used as received unless noted. Flash chromatography was performed using 40−63 μm silica gel. All NMR spectra were recorded in CDCl3 unless otherwise noted using both 300 and 400 MHz instruments. Polymer Brush Fabrication. PFPA was synthesized following previously reported methods.29 It was further purified using a plug of neutral alumina with DCM as eluent to remove any residual acrylic acid. The AIBN-silane initiator was also prepared using previously reported methods and, after synthesis, stored immediately in an inert atmosphere glovebox.30 Silicon wafers and glass microscope slides were cut using a diamond scribe and cleaned by sonication in hexanes, isopropyl alcohol, acetone, and deionized water for 1 min each. The slides were then cleaned using argon plasma for 5 min (Harrick Plasma PDC-32G) and subsequently placed in a slide stainer and transferred to an inert atmosphere glovebox (MBraun Labstar). A 10 mM solution of the AIBN-silane initiator was prepared in 20 mL of dry toluene in a scintillation vial. The vial was shaken vigorously to ensure full dissolution of the initiator and added to the slide stainer for 16 h. After 16 h, the initiator solution was removed and replaced with fresh toluene for storage until use. PFPA was degassed by bubbling argon through a needle at 0 °C and transferred to an inert atmosphere glovebox. An initiator substrate was sonicated in fresh toluene to remove any physisorbed material. The substrate was then dried under a stream of nitrogen, cut into smaller pieces, and placed 10 mL vials, and transferred back into the glovebox. To the vials, 71.6 μL of dry dioxane and 291.4 μL of PFPA were added and subsequently Teflon taped and capped. The vials were removed and placed in a UV reactor for 2 h. After 2 h, the vials were removed from UV irradiation and sonicated in THF to remove the substrate from the glassy polymer formed. The glassy polymer was saved for subsequent analysis via gel B
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Langmuir microcapillary printing (μ-CaP) using 250 μm channels. The solution was allowed to evaporate fully. The PDMS stamp was then removed, washed with DMF, and dried under a stream of nitrogen. PEFABLOC (40 mM) in DMF with 80 mM TEA was printed perpendicular to the printed ODIBO-amine via μ-CaP. The solution was allowed to evaporate, and then the stamp was removed, washed with DMF, and dried under a stream of nitrogen. The slide was then placed in a solution containing 0.006 mmol AMP, 0.012 mmol TEA, 0.1 mmol fluorescein-TBDMS, 0.02 mM TBD, and 9.8 × 10−4 mmol Texas Red azide for 30 min in 450 μL of dry DMF. After 30 min, the substrate was washed and sonicated in DMF and dried under a stream of nitrogen. The substrate was then characterized via fluorescence microscopy. Azide Coated Nanoparticle Synthesis. Octadecene coated nanoparticles were prepared by thermal decomposition according to the procedure of Cowger et al.33 A volume of 1 mL of the crude reaction mixture was dissolved in 3 mL of hexane then added to 20 mL of EtOH, and centrifuged down at 4400 rpm. The supernatant was decanted and the process repeated 2× to yield purified octadecene coated iron oxide nanoparticles. The purified particles were dissolved in 2 mL of DCM, and 50 μL of 11-azidoundecanoic acid was added. The reaction was shaken thoroughly then allowed to sit for 3 days. After ligand exchange, 20 mL of MeOH was added, and the particles centrifuged down at 4400 rpm. The supernatant was decanted and the process repeated 2× to yield purified 11-azidoundecanoic acid coated iron oxide nanoparticles. The presence of the azide peak at 2099 cm−1 was confirmed by FTIR. Trimorphological Surface. A grafted from poly(PFPA) brush on silicon was patterned with ODIBO and PEFABLOC. The substrate was then placed in a vial containing azide-functionalized iron nanoparticles in DCM for 5 min and continuously shaken. After 5 min, the substrates were removed, vigorously washed with DCM, and dried under nitrogen. The substrate was then creased using a PDMS stamp and Jeffamine M-2070 using our previously reported methods.32 Characterization. Fluorescent images were taken using a Nikon Eclipse NI-U, using a 10× objective lens. The filter sets used were a DAPI filter set (395/460), GFP filter set (488/509), and a red filter set (560/590) for AMP, fluorescein, and Texas Red, respectively. The creased morphologies of the substrates were collected using the ScanAsyst program on a Bruker Multimode atomic force microscope (ScanAsyst-AIR, k = 0.4 N/m, resonant frequency (f 0) = 50−90 kHz), and Nanoscope Analysis Software (Bruker) was used to analyze the nanoparticle topography images. The infrared spectra of the substrates were taken using a Nicolet model 6700 with a grazing angle attenuated total reflectance accessory at 256 scans with a 4 cm−1 resolution. The thicknesses of the polymer brushes were measured using a M-2000 spectroscopic ellipsometer (J.A. Woollam Co., Inc.) with a white light source at three angles of incidence (65°, 70°, and 75°) to the silicon wafer normal. The data were modeled using a Cauchy layer fitting both the extinction coefficient and refractive index for the polymer brush layer. Contact angle measurements were collected using Krüss DSA 100 using a 1 μL drop of 18 mΩ water (pH = 7) and averaging the results over three experiments. Raman spectra of the three distinctly functionalized regions were acquired using a confocal Raman microscope (InVia, Renishaw, Inc., Gloucestershire, United Kingdom). A 632.8 nm HeNe laser excited the sample through a 50× objective (N.A. = 0.75) with ∼3.1 mW of power (as measured at the sample). The resulting spot size had a diameter of 1−2 μm. Spectra between 3100 and 1000 cm−1 were acquired with a 30s acquisition time and 10 accumulations.
Table 1. Thicknesses and Contact Angles of Individual PEFABLOC, ODIBO-Amine, and Poly(PFPA) Substrates before and after Functionalization and Reaction with Fluorescent Dye poly(PFPA) brush AMP PEFABLOC ODIBO
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RESULTS AND DISCUSSION As a base layer, poly(PFPA) brushes were synthesized by surface initiated photopolymerization from azo-initiator containing silane monolayers using previously reported methods on Si/SiO2 substrates with an average thickness of 48.9 nm and a grafting density of 0.093 chains/nm2.30 Each postpolymerization modification reaction was first carried out on individual substrates to both ensure full functionalization and characterize
47.13 nm, 118.0° 44.49 nm, 116.1° 55.20 nm, 120.9°
reactive molecule
dye
90.26 nm, 100.0° 262.46 nm, 91.3°
163.54 nm, 86.5° 244.79 nm, 90.4° 407.72 nm, 87.2°
Figure 1. Infrared spectra of poly(PFPA) brushes functionalized with (A) ODIBO-amine, (B) PEFABLOC, and (C) AMP as well as ODIBO-amine clicked with Texas Red azide and PEFABLOC clicked with TBDMS fluorescein methyl ester.
the rate and extent of each reaction individually. The poly(PFPA) brushes were reacted with a solution containing 40 mM 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochlorC
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ide (PEFABLOC) in DMF with 80 mM TEA and 40 mM ODIBO-amine in DMF with 80 mM TEA. The substrates were removed and washed with DMF, dried under a stream of nitrogen, and characterized via spectroscopic ellipsometry, drop shape analysis, and grazing incidence attenuated total reflectance Fourier transform infrared spectroscopy (GATRFTIR). A change in thickness and contact angle occurred for each substrate, and are reported in Table 1. The FTIR spectra also indicated full conversion, resulting in the loss of the characteristic PFPA CO ester stretch at 1785 cm−1 and C−C aromatic stretch at 1523 cm−1 as well as the appearance of amide peaks at 1640 and 1540 cm−1 in each spectra (Figure 1). Once the individual functionalizations were confirmed, a patterned surface combining the two reactions outlined above was fabricated. To generate a patterned trireactive surface, R-μCaP was performed on the poly(PFPA) brush.25 PDMS stamps with 250 μm channels were fabricated using conventional lithographic methods, sonicated in acetone, dried in a vacuum oven at 45 °C, and further cleaned using Scotch tape. The stamps were placed directly on the brushes and 2 μL of 40 mM of ODIBO-amine in DMF with 80 mM TEA was wicked into the channels by capillary action. The solvent was allowed to evaporate, and the stamp was removed. Another PDMS stamp was then placed perpendicular to the initial pattern, and 2 μL of 40 mM solution of PEFABLOC was wicked into the stamp. The solvent was again allowed to evaporate, and the stamp was removed from the substrate, resulting in a surface that contained ODIBO (strained alkyne), PEFABLOC (sulfonyl fluoride), and activated ester functionality in a checkerboard pattern. To confirm the fidelity of the process, Raman spectroscopy was taken in each region using a Raman microscope (Figure 2). The spectra obtained in each area of the slide resulted in characteristic peaks for each reactive group. In the ODIBO-functionalized region, peaks were observed at 3067 cm−1 corresponding to the C−H aromatic, 2151 cm−1 (CC alkyne), and 1605 and 1564 cm−1 (CC aromatic) were observed. In the PEFABLOC region, peaks were observed at 3061 cm−1 (C−H aromatic), 1597 cm−1 (C C aromatic), and 1376 cm−1 (SO). Finally, in the PFPA region, peaks were observed at 1788 and 1652 cm−1, corresponding to the CO and CC aromatic stretches. These characteristic peaks for each functionality were not observed in the other areas of the substrate, indicating reasonable fidelity with no indication of cross-contamination. To further confirm the fidelity and orthogonal reactivity of these surface patterns, dyes containing the complementary click functionality were used to generate patterns that could be characterized using fluorescence microscopy. Individual reactions with dyes were first carried out on functionalized brushes and subsequently characterized via GATR-IR and DSA to confirm quantitative conversion. Individual ODIBO and PEFABLOC slides were clicked via SPAAC with Texas Red azide and SuFEx with a TBDMS fluorescein methyl ester, using a triazabicyclodecene (TBD) catalyst. Also, a poly(PFPA) brush slide was functionalized 40 mM aminomethylpyrene (AMP) with 80 mM TEA in 2 mL DMF with stirring. After 30 min, the substrates were washed with DMF, dried under a stream of nitrogen, and characterized. The contact angles, thicknesses, and FTIR spectra reveal successful and nearquantitative conversions for both reactions (Table 1; Figure 1). Each dye reaction was allowed to stir for 30 min due to the observed kinetics of the reaction of an aryl sulfonyl fluoride and an aryl TBDMS derivative, which has a pseudo-first-order rate
Figure 2. Raman spectra of poly(PFPA) (black), ODIBO (red), and PEFABLOC (blue) areas on a trireactive surface.
Figure 3. Pseudo-first-order kinetic plots of aminolysis (blue),26 SPAAC (red), and aryl silyl ether SuFEx (black) are shown.
Figure 4. Fluorescence microscopy images for the trifunctional surface of AMP, Texas Red Azide, and TBDMS fluorescein methyl ester: (A) DAPI filter set (395/460) exciting AMP, (B) red filter set (560/590) exciting Texas Red, (C) GFP filter set (488/509) exciting fluorescein, and (D) combined image of all three filter sets. D
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Figure 5. AFM topography images of a reacted trifunctional surface: (A) creases fabricated through μCP with Jeffamine-M2070, (B) azide coated iron oxide nanoparticles clicked to ODIBO, and (C) unfunctionalized, smooth poly(PEFABLOC amide).
constant of k′ = 0.0014 s−1 at 25 °C (Figures 1A and 3), an order of magnitude slower than the k′ = 0.04 s−1 observed between alkyl sulfonyl fluorides and an alkyl TBDMS derivative in our previous work.29 Although SPAAC (k′ = 0.0418 s−1 at 25 °C) and aminolysis (k′ = 0.2783 s−1 at 40 °C) are faster reactions, to closely mimic the one-pot orthogonal reaction with the trireactive surface, each slide was placed in the respective dye solution for a full 30 min. Since sulfonyl fluorides are known to react with amines at raised temperatures, a PEFABLOC functionalized surface was immersed in an AMP/ TEA solution, where no functionalization with AMP was observed. The true orthogonality of the surface chemistries was demonstrated using a one-pot, self-sorting reaction for postpolymerization modification. A trireactive surface was placed in a vial containing AMP, triethylamine (TEA), Texas Red azide, TBDMS fluorescein methyl ester, and TBD in 450 μL of DMF. The substrate was allowed to react for 30 min with agitation. It was then removed, washed thoroughly with DMF, and dried under stream of nitrogen. The patterned surface was then examined under a fluorescent microscope (Nikon Eclipse NI-U; 10×), using DAPI filter set (395/460), GFP filter set (488/509), and a red filter set (560/590) to excite AMP, fluorescein, and Texas Red, respectively. The obtained fluorescent images are shown in Figure 4. Figure 4A was obtained using the DAPI filter set and reveals the AMP derivatization; Figure 4B was obtained using the red filter set, where Texas Red is observed; and Figure 4C is the image obtained using the GFP filter set, which reveals the fluorescein substitution. Fluorescence in the AMP region was also observed using the GFP filter set likely due to the fact that densely packed AMP molecules lead to excimer formation.34 Excimers occur in pyrene-containing materials as a result of a ground state pyrene ring and an excited state pyrene ring in close proximity forming an excited state dimer. In this system, the AMP functionalized polymer brushes are densely packed, resulting in conditions that promote excimer formation, which is observed at 484 nm.35 A superposition of the images, shown in Figure 4D, illustrates three distinct regions with no noticeable cross-contamination, which corroborates the data obtained using Raman spectroscopy and demonstrates that sequential R-μCaP results in a patterned surface that self-sorts with high fidelity. To further expand the utility of this technique, a surface with patterned morphologies was fabricated using a trireactive template. The trireactive surface, fabricated in the same fashion described above, was placed in a vial of azide-functionalized iron oxide nanoparticles (∼20 nm) in DCM for 5 min with
constant stirring. After 5 min, the substrate was washed with DCM and dried under a stream of nitrogen. Then, using microcontact printing (μCP) with Jeffamine-M2070 (40 mM in toluene with 80 mM TEA), creases were generated in the remaining PFPA area of the pattern using our recently reported method.32 Analysis was then performed using atomic force microscopy (AFM) (Bruker ScanAsyst) which is shown in Figure 5. Figure 5A shows the topography of the PFPA brush, which was creased using μCP under confinement.32 In Figure 5B, the azide-functionalized nanoparticles were observed only in the ODIBO-functionalized channel region. The average diameter of the surface bound nanoparticles (21.96 nm) is consistent with the diameter of the source particles. Finally, in Figure 5C, the PEFABLOC channel is shown, where a smooth, featureless morphology with a RMS roughness of 3.55 nm was observed, containing neither nanoparticle or crease contaminants. To our knowledge, this is the first example of using click chemistry to generate three distinct nanoscale morphologies using orthogonal postpolymerization, without cross-contamination or lithographic methods.
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CONCLUSIONS This work demonstrates a versatile method to create an orthogonal, self-sorting surface in which three different chemical and/or morphological functionalities can be generated from a complex one-pot reaction. The reaction conditions are compatible with environmental oxygen and water, and are highly functional group tolerant. These surfaces offer a wide array of possibilities as a scaffold in the fields of biological and biomedical engineering and provide a method to fabricate different localized morphological and molecular environments on a single substrate, with the ultimate goal of more closely mimicking the complexity observed in biological interfaces.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01591. Experimental details; AFM topography images of unfunctionalized and functionalized poly(PFPA) brushes (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]. E
DOI: 10.1021/acs.langmuir.6b01591 Langmuir XXXX, XXX, XXX−XXX
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†
K.B. and J.Y. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Jin Xie and Taku Cowger for providing the starting iron oxide nanoparticles used. This work was supported by the National Science Foundation (DMR 0953112 and 1011RH252141, NSF GRFP).
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DOI: 10.1021/acs.langmuir.6b01591 Langmuir XXXX, XXX, XXX−XXX
